CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenue	$ 18,943
Type of revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 12,415	$ 4,173
General and administrative	5,156	1,368
Total operating expenses	17,571	5,541
Income/(loss) from operations	1,372	(5,541)
Other income (expense):
Interest income	49	378
Other income (expense), net	(2)
Net Income/(loss)	1,419	(5,163)
Net income/(loss) attributable to noncontrolling interest	1,489	(190)
Net loss attributable to common stockholders of Cullinan	$ (70)	$ (4,973)
Net loss per share, basic and diluted	$ 0.00	$ (0.27)
Total weighted-average shares used in computing net loss per share, basic and diluted	41,977,336	18,747,004
Comprehensive income/(loss):
Net income/(loss)	$ 1,419	$ (5,163)
Unrealized gain/(loss) on investments	(58)	30
Comprehensive income/(loss)	1,361	(5,133)
Comprehensive income/(loss) attributable to noncontrolling interest	1,489	(190)
Comprehensive income/(loss) attributable to Cullinan	$ (128)	$ (4,943)